Abstract: A welder having first and second electrodes configured to clamp and weld a fastener to a workpiece at an opening in the workpiece, wherein the first electrode is configured to move in a direction transverse to a clamping direction to center the fastener with the opening. The welder can carry out a method of aligning and welding an internally-threaded nut to a panel at an opening in the panel. The method includes the steps of: centering the opening in the panel relative to a first electrode using a tapered guide pin that extends into the opening; centering the nut relative to the opening using the tapered guide pin; clamping the nut and panel between the first electrode and a second electrode; and welding the nut to the panel by passing current through the nut and panel from one of the electrodes to the other electrode.